Conventionally, there has been examined a service of distributing information to vehicles using wireless communication technologies such as wireless LAN and DSRC (Dedicated Short Range Communications).
When the wireless LAN or the DSRC is used, a plurality of vehicles may be serviced within one wireless cell. In such a case, it is necessary to provide as many transmission sessions as services, thus degrading the efficiency of using wireless transmission channels.
A technology to solve this problem is disclosed in patent document 1 (JP-A-2001-189665 or U.S. Pat. No. 6,307,487). This technology outputs a meta-packet from an input file as a data aggregate. Here, the meta-packet is defined as a sequence of fragment data having information independent of each other. FIG. 23 shows relationship between an input file and a meta-packet to be generated. The following description is not a quotation from patent document 1. The inventors reproduced the contents using equations based on technical examinations. An input file 51 is divided into source packets s0, s1 . . . , and sn−1 each having a unit data length. A vector comprises these n source packets as components. The vector is multiplied by a coded matrix comprising n+k rows and n columns, where n and k are natural numbers. This multiplication yields a vector comprising n+k components. Each component is a meta-packet.
The coded matrix is formed so that a matrix of (1+α)n rows and n columns extracted from any (1+α)n rows becomes regular. Value α is sufficiently smaller than 1. Accordingly, obtaining the necessary number of meta-packets, i.e., (1+α)n meta-packets makes it possible to restore the input file, i.e., the vector comprising source packets s0, s1 . . . , and sn−1. To do this, an inverse matrix comprising (1+α)n rows and n columns is generated by extracting rows corresponding to the obtained meta-packets from the coded matrix. The inverse matrix is then multiplied by a vector comprising the obtained meta-packets. FIG. 24 schematically shows a method of restoring an original input file from the obtained meta-packets.
This technology can be used for the wireless LAN and DSRC. When meta-packets are continuously transmitted in a wireless cell, information can be simultaneously distributed to vehicles that enter and exit from the wireless cell. When entering the wireless cell, a vehicle repeatedly receives transmitted meta-packets until the necessary number of meta-packets is accumulated. When the necessary number of meta-packets is accumulated, an original file can be restored based on the accumulated meta-packets. As mentioned above, any meta-packets can be received only when the necessary number of meta-packets is available. It is unnecessary to adjust the timing to start a service between a service provision party (sender) and a service receiving party (recipient). Even when a plurality of vehicles enters or exists from the wireless cell, each vehicle can complete the reception when the necessary number of meta-packets can be obtained. Therefore, it is unnecessary to provide a session individually.
When the information distribution service is actually provided for vehicles, however, it is not always the best way to distribute the same file to all vehicles in one wireless cell. Information to be distributed may often vary with vehicle's situations such as a traveling speed.
Further, the same transmission channel is used to distribute the information to a plurality of users. Therefore, it is necessary to devise a method of preventing the other users from intercepting the data that should be distributed to a specific user.
Patent document 1 does not disclose a means for solving these situations by using the above-mentioned meta-packet technology.